Mixed bedrock-alluvial rivers can exhibit partial alluvial cover, which
may play an important role in controlling bedrock erosion rates and
landscape evolution. However, numerical morphodynamic models generally
are unable to predict the pattern of alluviation in these channels.
Hence we present a new two-dimensional depth-averaged morphodynamic
model that can be applied to both fully alluvial and mixed
bedrock-alluvial channels, and we use the model to gain insight into the
mechanisms responsible for the development of sediment patches and
patterns of bedrock alluviation. The model computes hydrodynamics,
sediment transport, and bed evolution, using a roughness partitioning
that accounts for differential roughness of sediment and bedrock,
roughness due to sediment transport, and form drag. The model
successfully replicates observations of bar development and migration
from a fully alluvial flume experiment, and it models persistent
sediment patches observed in a mixed bedrock-alluvial flume experiment.
Numerical experiments in which the form drag, sediment transport
roughness, and ripple factor correction were neglected did not
successfully reproduce the observed persistent sediment cover in the
mixed bedrock-alluvial case, suggesting that accounting for these
different roughness components is critical to successfully model
sediment dynamics in bedrock channels.